This invention relates generally to signal generators and in particular to a signal generator having a flexible architecture for generating test signals.
Wireless communications, including digital cellular telephones and personal communications service (PCS) telephones, are rapidly becoming a major sector of the communications industry. Emerging wireless communications technologies are a diving force behind the rapidly expanding number of complex signals such as digitally modulated signals that must now be accommodated by the signal generator, both in the design and manufacturing environments. The wireless system designer who must choose a particular digital modulation is faced with a number of challenges. The wireless system must allow for signal strengths that vary over time and location, with multipath, fading and interference. Wireless handsets are increasingly smaller in size and with limited battery capacity. At the same time, user demands continue to increase for higher data rates, better voice quality, fewer dropped calls, and longer talk times. Designing, manufacturing, and maintaining wireless devices thus requires appropriate test equipment such as signal generators which are capable of generating test signals for precisely simulating real world conditions as well as known signals that confonn to industry standards. A discussion of various digitally modulated signal types that exist may be found in "Digital Communication, Second Edition", Lee, Edward, A. and Messerschmitt, David G., Kluwer Academic Publishers, Massachussetts, 1994.
Signal generators must be versatile and powerful enough to handle both existing and newly created modulation types. During the development of a new wireless system, designers may be faced with the problem of not having a receiver to verify the operation of the signal generator and no signal generator capable of verifying the operation of the receiver. A versatile signal source will help in avoiding this dilemma by providing a known test signal by which the performance of the new receiver can be determined without developing specialized prototypes.
Signal generators may be used to test wireless communications devices in a number of ways. For example, it may be useful to examine the device's response to non-ideal signals by providing a test signal that deviates in a known manner from an ideal signal. As a further example, wireless communications devices must operate in crowded spectral environments alongside other communications systems. A number of signal generators may be combined in parallel to simulate in a controlled manner a complete spectral environment by generating a multitude of interfering signals having a variety of signal strengths, frequencies, and modulation types.
In the manufacturing environment, the same test station must often handle different types of wireless devices having a variety of modulation types. A versatile signal source reduces the need for additional equipment and simplifies test system requirements. Function generators are well known for their versatility in providing test signals which are amplitude, frequency, or phase modulated over frequency ranges typically spanning d.c. (direct current) to approximately 20 MegaHertz (MHz). Function generators are well suited for testing analog devices and simulating traditional analog modulation types and have the advantage of being able to accept an input signal and generate a test signal with real time modulation responsive to the input signal. However, the ability of the function generator to generate more complex digital modulation types is very limited because of its primarily analog architecture.
Arbitrary waveform generators (AWGs) are a more recent design, employing digital waveform memories and digital to analog converters (DACs) to generate a test signals of greater complexity. Given an adequate memory size and maximum sample rate, AWGs can be very versatile in simulating a variety of test signals. However, AWGs generally have no ability to accept digital input signals in order to generate real-time signals that communicate actual information. The test signal being generated by the AWG must be calculated and stored ahead of time as pre-computed samples in the digital waveform memory. Because pre-computed samples are stored in terms of voltage values to be played back at a selected sample rate, the computations necessary to generate a desired test signal can be significant.
The ability of the AWG to generate complex signals is further limited by the size of the digital waveform memory and the maximum sample rate, often requiring trade-offs between frequency content and waveform complexity. Because the digital waveform memory is typically used to generate the test signal by continuously repeating the contents of the digital waveform memory, care must be taken to avoid a discontinuity between pre-computed sample values stored at the beginning and the end of the digital waveform memory. This potential discontinuity further limits the ability of the AWG to generate the desired test signals.
It is generally understood that traditional digital signal processing (DSP) chips, which are essentially microprocessors optimized for signal processing applications, are capable of generating test signals when properly programmed and coupled to a suitable DAC and supporting hardware devices. The DSP chip can accept a real-time input signal, typically in the form of a digital data stream, to produce a real-time digitally modulated signal, with the signal processing calculations handled real-time by the DSP chip. Because of the relative high complexity required to calculate each output sample of the digitally modulated signal in real time, the signal bandwidth that may be obtained is limited by the throughput of the DSP chip and surrounding hardware. Furthermore, reconfiguring a signal generator that is implemented using a DSP chip to work with any of a variety of digital modulation types is difficult and time consuming.
Therefore, it would be desirable to provide a versatile signal generator for generating a variety of test signals including digitally modulated signals with real-time modulation. It would be further desirable that the versatile signal generator be implemented using only dedicated hardware integrated circuits and memory which can be readily configured for different test signals.